I’m aware it’s archaic, but good culture and passion for your jersey makes moving to your current clubs bitter rivals harder.
if you look at the 120+ year history of Liverpool and Man Utd in the EPL, a competition where players transfer constantly and loyalty is extremely rare - only 8 players who played for Liverpool have ever then pulled on a United shirt, and of that only 5 of them were direct transfers from one club to another - the last one being 83 years ago. Why? Because the culture is if you’re a red, you hate Man U. It’s ingrained. Even for players who only spend a season or two at Liverpool.